1. Field of the Invention
This invention relates to apparatus and method for determining the rate of flow of a fluid by measuring an electrical potential difference developed in the fluid as the fluid moves through a magnetic field.
2. Background Information
In a magnetic flow meter an electrical potential difference developed in the fluid is sensed by at least one pair of electrodes contacting the liquid and spaced apart from each other along a line that is generally orthogonal to both the direction in which the flow is being measured and a magnetic field produced by a magnet. The measured potential difference has a magnitude proportional to the flow rate of the fluid. As is known to those skilled in the art, the overall potential difference between two such electrodes, usually termed a voltage difference, has two major components: a) a flow-related voltage due to the flow of the fluid when acted upon by the magnetic field; and b) a net xe2x80x98drift voltagexe2x80x99, which is the sum of voltages due to all other factors, such as electrode polarization.
In prior art flow sensors of this type, alternating magnetic fields from electromagnets have generally been used to provide an alternating magnetic field. The alternating magnetic field facilitates signal amplification and processing that accepts flow-related electrode signals while rejecting electrode drift signals which would otherwise introduce serious measurement errors. However, generating those fields and processing the measured voltage signals requires sophisticated circuits and techniques which raise the cost of such sensors and limit their application.
An example of a radical departure from the prior art is found in my U.S. Pat. No. 6,085,599 in which I teach mechanical means to alternate the polarity of the magnetic fields. Those techniques provide practical ways of simplifying magnetic flow sensors and reducing their costs. However, the use of mechanical means to alternate the field polarity, even though this may be performed with a high degree of ruggedness and reliability, reduces the marketability of such an instrument The disclosure of U.S. Pat. No. 6,085,599 is incorporated herein by reference.
Another problem encountered in prior art magnetic flow sensors is that of entrapment of ferromagnetic debris. This is particularly true of arrangements using permanent magnets as in my U.S. Pat. No. 6,085,599. Such debris can change the magnetic flux distribution and thereby alter the calibration of the flow meter. Moreover, pieces of ferromagnetic debris can sometimes bridge the electrodes, which are normally electrically insulated from each other, producing a conductive path that may partially short out the electrode signals and thereby reduce the output voltage. Fine particles of debris can also form a film on normally insulating portions of the structure surrounding the electrodes and thereby shunt the electrode signals.
It is therefore an object of the invention to provide a practical magnetic flow sensor using stationary permanent magnets.
It has also been discovered that the methods of the present invention can be used with conventional magnetic flow sensors using electromagnets to improve their performance and such is therefore a further objective of the invention.
The above and other objects are attained by magnetic flow sensors in accordance with various preferred embodiments of the present invention. In preferred embodiments the magnetic axis (i.e., the line extending from the south to the north pole) of a permanent magnet is oriented generally perpendicular to a direction of flow of a fluid. As is known in the magnetic flow metering art, the flux from a magnet arranged in this fashion generates, in the fluid, a voltage difference proportional to the flow rate of the fluid. In various embodiments of the invention this voltage difference is sensed by the use of a sensing head comprising a pair of electrodes (which preferably have the same size and shape and are made of the same material) which are spaced apart from each other along a line that is generally orthogonal to both a direction of flow and the magnetic axis.
The voltage indicative of flow rate is measured when the two electrodes of a pair are in an open-circuit state in which they are externally electrically connected to a high impedance voltage measurement circuit. In this open circuit state the electrode potentials are electrically influenced by electrode polarization and other measurement error-inducing factors that develop relatively slowly. In order to minimize measurement errors with these factors, sensors of some embodiments of the invention provide an operating cycle in which the two electrodes of a pair thereof are in a closed circuit state for most of the time, and are placed in an open circuit state only during a brief measurement interval portion of the operating cycle. When in the closed circuit state the electrodes may be short circuited to each other, connected to respective reference voltage sources (typically zero to a few tens of microvolts) or connected to a common potential such as ground. A major purpose of the closed circuit state, reducing drifts, is served by connecting the two electrodes together. Connection to other selected potentials, including ground, can provide compensation for minor drifts. The reference voltage sources include voltage levels which may be different for each electrode and which may even vary with the output flow rate signal from the flow sensor. In the closed circuit state, particularly during installation and set-up, the electrodes may be connected to alternating potentials having magnitudes as high as several volts and frequencies of several kilohertz in order to drive the electrodes quickly into a steady state condition. Periodically, each electrode pair may be switched from its closed circuit to its open circuit state for a brief time interval so that the flow-generated voltage difference then appearing at the electrodes may be detected and processed to provide an output signal representative of the flow rate of the fluid. During the open circuit portion of this duty cycle, drift inducing factors begin to cause drift signals to develop. However, they develop relatively slowly compared to the brief time interval required to detect the flow rate signal and thereby enable electronic processing to discriminate between the two. This method of flow rate detection thereby enables an extremely simple magnetic flow sensor to be made. In other cases, in which the flow rate signal is found to change slowly with respect to drift signals, the closed circuit state may comprise a smaller portion of the operating duty cycle and the open circuit state a correspondingly large portion of the duty cycle so as to allow the full magnitude of the flow rate signals to be detected.
As will be disclosed in greater detail hereinafter, the flow rate of a fluid can be sensed by arrays of sensing heads comprising two or more pairs of electrodes and at least one magnet having its magnetic axis oriented perpendicular to a direction of flow. Each of the sensing heads in an array, as recited above, comprises a pair of electrodes spaced apart from each other along a line generally orthogonal to both the direction of flow at that sensing head and to the magnetic flux. The sensing heads in an array thereof are spaced apart from each other along the flow path of the fluid. For example, two sensing heads can be spaced out along a section of pipe or tubing. The flow rate voltages from the plurality of heads can be polarized to be additive in the associated signal processing circuitry, which may be adapted to measure all the heads simultaneously, or which may measure the voltages one at a time in a sequential, scanning, fashion. Furthermore, because more than one pair of electrodes may be used with a single or with cooperative magnetic fields, the sensor can be configured as comprising paired arrays of electrodes that can be momentarily externally connected in differing combinations so as to provide a statistical sampling base from which the output signal is derived. For example, if two arrays of four electrodes each are paired, sixteen different combinations of individual electrode pairs can be sampled. Because DC drift voltages at the various electrodes would have a random distribution of magnitudes and of polarities, the drift voltages thus tend to average out to zero when the overall electrode voltages are summed or sampled and averaged. The magnitude of the flow related signal can thus be made relatively high compared to the error related drifts, thereby improving sensor performance. Series connection of the electrodes between more than one sensing head is also applicable and similarly advantageous and enables the direct addition of the flow related signals to be obtained. The present invention is well adapted to such configurations because of the low cost of the components that are used.
In addition to improving the ratio of flow-related signals to drift signals, a two-headed sensing configuration comprising an upstream head and a downstream head can be used to detect the presence of ferromagnetic debris, most of which is likely to be trapped by the permanent magnet portion of the upstream sensing head. This debris can alter the magnetic flux distribution and shunt the flow-related voltage of the upstream head, thereby reducing the magnitude of its flow-related voltage. Thus, if one compares the flow-related signals from identical upstream and downstream sensing heads and finds that those signals differ by more than some predetermined threshold value, one can conclude that at least the upstream head is contaminated with ferromagnetic debris and that cleaning of the wetted portions of the sensor is required.
Although various numbers of sensing heads can be used in the invention, in preferred methods of operation the paired electrodes of each sensing head are in the closed circuit state during a relatively long portion of an operating duty cycle. During a relatively short portion of the duty cycle a switching device can be used to sequentially open circuit pairs of electrodes and connect each open circuited pair to a common measurement circuit in order to measure its flow-related open circuit voltage. A switching device can also open circuit pairs of the electrodes and connect them to separate inputs of a common measurement circuit to measure the flow related voltages. Those skilled in the signal processing arts will realize that with these and other arrangements for aggregating open circuit voltages one can obtain a simple average of the output voltages, an average of the sum of the individual output voltages, or various other selected statistical measures.
Generally speaking, the flow-generated component of the open circuit voltage will appear quickly (i.e., it can be measured after a predictable rise time that depends primarily on the resistivity and dielectric constant of the flowing fluid) after an electrode pair is switched from a closed circuit state to an open circuit state. Electrode pair drift voltages, by contrast, depend on electrode polarization and other generally much more slowly acting effects and can thus generally be effectively excluded by making the open circuit voltage measurement quickly. Thus, one can readily determine a fluid-dependent operating duty cycle comprising a first period in which all electrode pairs are connected together in a closed circuit state for a long enough interval for polarization and other drift effects to reach an acceptably stable condition; and a second readout period in which appropriate switching devices and voltage measurement circuitry are used to detect the open circuit voltages from all the electrode pairs used in the sensor. In a preferred embodiment the first period is substantially longer then the second. Other relationships between the lengths of the first and second periods are also workable.
In some embodiments multiple permanent magnets are used with an internal streamlined body and a flow tube, both of which are electrically insulating and in contact with the fluid. Each such section has its own pair of electrodes. Both the magnetic flux, which is orthogonal to the fluid flow, and the fluid itself are thus concentrated to provide a relatively large flow-related signal.
In another preferred embodiment of the present invention, the flux from two permanent magnets reinforce each other across an orthogonally oriented passage through which a fluid flows. Various other preferred embodiments including probe configured flow sensors are included.
In some embodiments, the present invention is applied to conventional magnetic flow sensors which use a pulse of electrical energy through a coil of wire to produce a pulsed magnetic field. After the pulsed magnetic field stabilizes, the electrodes are placed in the open circuit state so that the flow generated voltage difference can be detected and processed to provide a flow signal representative of the flow rate of the fluid. Operation is therefore essentially the same as when the permanent magnet is used.
Those skilled in the arts of magnetic flow sensing will appreciate that although relative motion between a liquid and a sensing head is essential in instruments of this sort, there is no requirement that the sensing head be stationary in an inertial frame of reference. One can equally well use the invention for measuring the rate of progress of a sensing head through a stationary fluid, as is done when measuring the speed of a ship having a sensing head mounted to or projecting outwardly from its hull. Moreover, one can configure a sensor having two pairs of mutually orthogonally disposed electrodes (e.g. as depicted in FIG. 1) in which each of the pairs is responsive to a component of fluid flow orthogonal to the line along which that pair is spaced. A sensor of this sort can be used to determine the direction of flow, as well as for measuring the magnitude of the flow rate.
Although it is believed that the foregoing recital of features and advantages may be of use to one who is skilled in the art and who wishes to learn how to practice the invention, it will be recognized that the foregoing recital is not intended to list all of the features and advantages. Moreover, it may be noted that various embodiments of the invention may provide various combinations of the hereinbefore recited features and advantages of the invention, and that less than all of the recited features and advantages may be provided by some embodiments.